Many considerations play a role in the selection of a system to drive elevator car doors in an elevator. One major constraint in elevators is space. The first space limitation is the length of the door system. The door operating system has to fit within the hoistway and thus, cannot exceed the width of the elevator car, which is frequently referred to as the elevator envelope. The second space limitation is the thickness of the door operating system. The door operating system has to be sufficiently narrow not to interfere with the hoistway as the elevator car travels up and down the hoistway. If the door system is too large and cannot fit between the elevator car and the hoistway, the door system will encroach on the elevator car space. An alternative to encroaching on elevator car space is to mount the door operating system on top of the elevator car. However, such a solution would invite other problems such as an increase in door rocking, restriction of overhead running clearance, and limited access to the car top. Therefore, the door operating system should not enlarge the car door envelope and should be sufficiently narrow not to interfere with the hoistway.
Another major consideration in selecting the elevator car door operating system is the cost of the system and the cost of maintaining and servicing the system. The size of the motor determines the cost of each unit. Also, the tight tolerancing required for some motor configurations increases the cost by demanding expensive precision manufacturing.
An additional constraint in the selection of the door operating system is weight. Since the door operating system is mounted directly on the elevator car and travels therewith, the door operating system should not be excessively heavy.
Once the door operating system meets the space, cost, and weight limitations, it must have a certain level of performance. The opening and closing of the elevator car doors must be simultaneous, smooth, and quiet.
In conventional elevator systems, elevator car doors are selectively opened and closed by a rotary electric motor driving a mechanical assembly, which typically includes a plurality of moving parts such as gear boxes, a set of drive arms, linkages and cams. The major drawback of the existing elevator car door system is that it is susceptible to misalignments and requires frequent adjustments that result in high maintenance costs. Also, the misalignments degrade the performance of the system such that door opening and closing functions are not consistently smooth.
An alternative to the existing mechanical door system is the linear motor driven door system. Although a number of patents have disclosed the use of linear motors on doors, implementation of linear motors in door systems in general, and in elevator car door systems specifically, has been very limited. Most of the existing patents have significant shortcomings and are not practical because they violate either space, cost, or weight constraints, or a combination thereof.
For example, U.S. Pat. No. 1,881,016 to Rose issued on Oct. 4, 1932 and entitled "Door Operating Mechanism" shows a door system driven by an induction motor. The patent teaches a motor secondary attached to one door and a motor primary attached to the second door. The major shortcoming of the disclosed configuration is that the excessively long motor secondary is free hanging and cannot be supported. Additionally, the unsupported motor secondary may buckle when compressed during the door opening function. Thus, the Rose patent does not provide a practical alternative to the existing door systems.
U.S. Pat. No. 1,881,014 to Ayers issued on Oct. 4, 1932 and entitled "Supporting and Operating Means for Doors" shows a door system with motor secondary attached to a door, and moving therewith, and a stationary motor primary. One major drawback of the disclosed configuration is that since the moving motor secondary overhangs the door on both sides, the door opening envelope is significantly enlarged. Therefore, the Ayers disclosure also does not represent a practical solution for modern elevator door systems.
A more recent U.S. Pat. No. 5,172,518 issued on Dec. 22, 1992 to Yoshino discloses an apparatus for doors using a linear motor. The patent shows two U-shaped motor primaries attached to a single door and a T-shaped motor secondary. One major reason for the disclosed configuration not being suitable for modern elevators is the thickness of the motors. Another major reason for the disclosed configuration not being implementable is that the open structural portion of the U-shaped motor primary may incur buckling from inherently high attractive forces.
Therefore, none of the existing patents provide a practical alternative to current mechanically driven elevator door system.